1. Field of the Invention
The present invention, according to a first aspect thereof, relates to a borehole radar tool for directionally sensitively locating transitions in the subsurface surrounding the borehole radar tool in use, comprising a generating assembly for generating the electromagnetic radiation having a frequency of between 10 MHz and 200 MHz, signal processing means for processing the received electromagnetic radiation, and a housing having a substantially cylindrical wall and a central axis which accommodates at least:
a transmitting antenna assembly for emitting the electromagnetic radiation generated by the generating assembly, comprising a transmitting antenna and an electroconductive reflector; and
a receiving antenna assembly for receiving the electromagnetic radiation reflected by the surrounding subsurface, comprising a receiving antenna and an electroconductive reflector,
wherein both the transmitting antenna and the receiving antenna extend parallel to the axis of the housing and both comprise a dipole antenna, each reflector comprising a reflective surface which extends abreast of and parallel to the respective dipole antenna at a distance therefrom, and the space between each reflective surface and that section of the housing which in each case is located opposite thereto being filled with a medium having a dielectric constant of at least 10.
2. Background Art
A borehole radar tool of this type is described in WO-A-89/03053. This publication describes a radar instrument for use in a borehole for locating fractures in a geological formation, comprising directionally sensitive transmitting and receiving antennae. Emitting means generate a pulsed radar signal with an output in the frequency range of between 30 and 300 MHz. This signal is radiated via a reflector, in particular a reflector consisting of two electroconductive plates arranged in the shape of a V. A receiving antenna, provided with a similar reflector, is used to intercept the radar signal reflected by the formation. The space through which the radiation propagates within the housing is filled with a barium titanate powder/air mixture, whose dielectric constant matches that of the surroundings and in particular has a value of about 80.
Using a borehole radar tool of this type, it is possible to achieve a certain directional sensitivity. With many applications there is a need for both the highest possible directional sensitivity and for as much radiation as possible penetrating into the subsurface. Given the constraints to which borehole radar tools are subject, including a preferred diameter of at most 20 cm and the electromagnetic characteristics of the subsurface to be surveyed, it has so far not proved possible to increase the directional sensitivity.
The object of the first aspect of the present invention is to provide a solution to the abovementioned problem, said aspect being characterized, to this end, in that the transmitting antenna and receiving antenna extend near the wall of the housing and in that at least that section of the reflector which is located diametrically opposite the transmitting antenna and receiving antenna, respectively, likewise extends near the wall of the housing.
Using a borehole radar tool of this type, it is possible to achieve a higher directional sensitivity in conjunction with high power of radiation penetrating into the subsurface. Two variables have to be considered in this context. The first variable is the ratio between the emitted power in the target direction and the emitted power in the other direction(s). This ratio should be as large as possible. The second variable is the total power emitted in the target direction. Obviously, this should be as large as possible. This is desirable to achieve the highest possible penetrative power. Thus a good image of the subsurface can be obtained with as small a number as possible of expensive boreholes.
A configuration according to the first aspect of the invention was found to function satisfactorily. In such an arrangement, the reflector should have a certain surface area. In practice, optimal dimensions exist for the reflector, which predominantly depend on the constrained dimensions of the housing.
The first aspect of the invention provides, in addition, for the distance from the transmitting and/or the receiving antenna to the reflective surface to be as large as possible. This is achieved by means of a borehole radar tool according to the first aspect of the invention. The term xe2x80x9cdistancexe2x80x9d in this context refers to the mean distance between the reflective surface and the transmitting or receiving antenna.
The term xe2x80x9cnear the wall of the housingxe2x80x9d in this context means that the relevant section of the reflector, or the centre of the transmitting and/or the receiving antenna, respectively, is located at a distance from the inner wall of the housing which is at most a quarter of the inside diameter of the housing. In particular, the distance between the inner wall of the housing and the relevant section of the reflector, or the transmitting and/or the receiving antenna, respectively, is at most 2 cm, advantageously less than 1 cm. Thus, as large a distance as possible between antenna and reflective surface can be achieved in a simple manner. In special conditions, for example at very high frequencies and when the apparatus is filled with a dielectric having a very high dielectric constant, the distance can alternatively be greater than 2 cm.
In a particular embodiment, at least one of the reflectors forms part of the wall of the housing. Thus it is possible for the dimensions of the housing, i.e. of the borehole, to be optimally utilized to the advantage of the reflector. For example, the housing is fashioned as a cylinder from a nonconductive material, for example plastic, part of the cylinder being formed by the reflector section which is fabricated from metal, for example.
In another embodiment, at least one of the reflectors comprises a thin plate. In this embodiment, a narrow space exists between the reflector and the housing, said space accommodating, for example, the cabling to the transmitting and/or receiving antenna. It is important for said cabling to be screened against electromagnetic radiation present in the space between the reflective surface and that section of the wall of the housing which is situated opposite thereto. The reflector can serve as such a screen.
The shape of the reflector, and in particular of the reflective surface, is not subject to any particular constraint. Suitable, for example, are two straight plates touching one another at one end, each end of both plates being situated near the wall of the housing. This affords a V-shaped reflector.
Advantageously, at least one reflective surface is a substantially smoothly curved surface which, as seen in the axial direction of the housing, at least substantially forms part of a conic. Giving the reflective surface such a shape makes it possible to ensure that the reflective surface at least largely follows the shape of the wall of the housing, thus making the mean distance from the reflector to the transmitting and/or receiving antenna as large as possible.
The term xe2x80x9csubstantially smoothly curvedxe2x80x9d in this context means that it is acceptable for the reflective surface to exhibit deviations in its shape whose dimensions are much smaller than the wavelength used, in particular at most {fraction (1/10)} of the wavelength of the electromagnetic radiation. Owing to the wave properties of the electromagnetic radiation, the reflective surface will then still appear to be smoothly curved. Examples of such deviations in shape are holes for wiring, or bending edges in a faceted reflector.
The conic is preferably chosen to be as advantageous as possible. Advantageously, the conic is a circle having a radius which is substantially equal to half the inside diameter of the housing. In this way, it is ensured that the mean distance between the reflective surface and the transmitting and/or receiving antenna is at its maximum. If the reflective surface is formed by the inner wall of the housing, the radius of the circle is equal to half the inside diameter of the housing. If the reflector is a thin plate near the inner wall of the housing, the radius of the circle is equal to half the inside diameter of the housing minus the thickness of the plate, possibly reduced by the distance between plate and inner wall of the housing. In particular, the last mentioned distance is less than 2 cm, advantageously less than 1 cm.
Advantageously, the angle formed by at least one of the reflectors is between 45xc2x0 and 180xc2x0. If said angle is between these two limits, a satisfactory emission characteristic is obtained owing to the reflective surface being large enough without too much radiation being lost via reflection of the emitted or intercepted radiation in a direction other than the desired one.
More advantageously, the angle formed by at least one of the reflectors is between 145xc2x0 and 155xc2x0. At these angles, an even better emission characteristic and consequently a better directional sensitivity is obtained.
In another preferred embodiment, the conic is a parabola. In particular, the focal length of the parabola is between 0.5 and 0.75 times the inside diameter of the housing. In many cases the centre of the transmitting and/or receiving antenna is in the focal point of the parabola, but it was found that it is more important for the distance between reflective surface and transmitting and/or receiving antenna to be as large as possible.
In the borehole radar tool according to the first aspect of the invention, the space between the reflective surfaces and that section of the wall of the housing which in each case is situated opposite thereto is filled with a dielectric. The function of the dielectric is, on the one hand, to shorten the wavelength of the electromagnetic radiation in the region between the antennae and the reflective surface. This results in an improvement of the collimating characteristics of the reflective surface. For this purpose, the dielectric constant of the dielectric is in principle chosen to be as large as possible.
On the other hand, the value of the dielectric constant of the dielectric at the transition towards the surrounding subsurface must match the value of the dielectric constant of that subsurface. The dielectric constant of, for example, the Dutch subsurface varies between xc2x15 (dry sand) and xc2x140 (wet sand, clay), and xc2x180 for water (e.g. groundwater). The smaller the difference in the dielectric constant between the subsurface and the dielectric, the larger the proportion of the generated electromagnetic radiation which actually penetrates the subsurface. This is desirable in order to be able to sample as large a proportion of this subsurface as possible by means of one measurement. With high penetrative power, measurements can extend to large distances, thereby requiring relatively few expensive boreholes. In such an arrangement, that fraction of said space which adjoins the wall of the housing can be filled with a suitable dielectric. In practice, the space between the reflective surfaces and that part of the wall of the housing which in each case is situated opposite thereto can be completely filled with one dielectric.
Preferably, the dielectric in the space between the reflective surfaces and that section of the wall of the housing which in each case is located opposite thereto has a dielectric constant of between 20 and 100, and more preferably between 60 and 100 and most preferably of about 80. At these values for the dielectric constant, a satisfactory compromise is achieved between suitable wavelength shortening for good collimating characteristics and a large proportion of radiation penetrating into the subsurface.
In a particular embodiment of the borehole radar tool according to the first aspect of the invention, the dielectric substantially comprises water. Of all the dielectric materials having a dielectric constant of about 80, water is particularly suitable, but e.g. barium titanate/air mixtures, as disclosed in WO-A-89/03053 will likewise be quite satisfactory.
The present invention, according to a second aspect thereof, generally relates to determining electromagnetic contrast information of an object.
With the aid of electromagnetic contrast information, it is possible to determine transitions in the electromagnetic properties of a site. Such transitions may, for example, indicate fractures or discontinuities, the presence of a material having different electromagnetic properties or other changes in the composition of the site.
The site referred to within the scope of the second aspect of the present invention is not subject to any restrictions in terms of its nature. In particular, however, site here refers to either (a section of) the ground or a pipeline at least partly made of metal. In the case of the ground, the electromagnetic contrast information observed can be used to locate buried objects or, for example, minerals such as oil, while in the case of the pipelines, the information can be used, for example, to determine where corroded patches are located and how thick these are, or where joints of pipe sections are located.
The determination of electromagnetic contrast information is often carried out with the use of geophysical imaging methods. Geophysical imaging methods frequently make use of a so-called subsurface measuring device. Subsurface measuring devices in this context refer to devices which are able to generate electromagnetic fields or to emit electromagnetic radiation which penetrates into the surrounding subsurface. Electromagnetic fields in this context should be understood to comprise either purely electric or alternatively magnetic fields. A fraction of the electromagnetic fields of radiation which is reflected by the subsurface and especially by the electromagnetic transitions located therein or, in the case of electromagnetic fields, is generated by induction in an underground electromagnetic transition or object and is then emitted, can be intercepted by the subsurface measuring device and be recorded in the form of a number of output signals. After a number of output signals have been collected, it is possible, by processing them, to obtain an image of the contrast characteristics of the surveyed section of the ground. Examples of subsurface measuring devices are subsurface radars, borehole radar tools and so-called NanoTEM measuring devices, which are known per se.
The measured data thus obtained can be divided into two groups. The first group is sometimes referred to as xe2x80x9cwave dataxe2x80x9d. These arise from situations in which the electromagnetic fields or radiation propagate within the surrounding subsurface in a wave-like manner. The electromagnetic energy then has a wave character.
The second group is sometimes referred to as xe2x80x9cdiffusion dataxe2x80x9d. These arise from a situation in which the electromagnetic field diffuses within the medium, as is also the case, for example, with heat. The electromagnetic energy then has a field character. In this case the field ensures that electromagnetic transitions or objects located in the subsurface will reflect or, via induction, emit radiation, in other words will form secondary field or radiation sources.
In processing wave or diffusion data, use is made of algorithms which likewise can be divided into two groups which, in principle, can both be applied to both wave data and diffusion data.
The first group of algorithms can be designated by the name xe2x80x9cinversionxe2x80x9d algorithms. This group of algorithms addresses the simultaneous determination of the shape and location of the object or the subsurface transition and of the unknown electromagnetic parameters.
The second group of algorithms is in fact referred to by the name xe2x80x9cimagingxe2x80x9d algorithms. This group of algorithms is able only to reconstruct the shape and the location of the electromagnetic transition. Particularly when these are applied to diffusion data, what is really determined is the shape and the location of the secondary field or radiation sources which in turn form an indication for the electromagnetic transitions in the subsurface. The processing speed and the mathematical stability of the imaging algorithms is greater than that of the inversion algorithms. The imaging algorithms are therefore used on a large scale.
The second aspect of the present invention relates, more in particular, to an algorithm for use in determining electromagnetic contrast information in a site according to the preamble of claim 12.
An algorithm of this type is often referred to by the name xe2x80x9cback-projectionxe2x80x9d algorithm and is described, for example, in U.S. Pat. No. 6,005,916.
The back-projection algorithm is widely used as an imaging algorithm in the processing of wave data. The back-projection algorithm essentially consists of the application, to measured data, of the adjoint integral operator linearized with the aid of Born""s approximation.
This algorithm is simple and therefore also efficient. However, it results in incorrect imaging if it is applied unmodified to diffusion data. The position of the object is not reproduced correctly, said position, in particular, being shown too close to the surface of the site, i.e., for example, too close to the earth""s surface or the outer surface of the pipeline.
For diffusion data, the back-projection algorithm can therefore not be applied in the conventional form. Use must be made of computation-intensive inversion algorithms. Imaging with the aid of such an inversion algorithm is much more sensitive to noise and, because of the computational capacity required, not (readily) possible in three dimensions.
It is therefore an object of the second aspect of the present invention to provide a solution to the abovementioned problem. This object is achieved with the aid of an algorithm as described in claim 12.
The E-function described in claims 12-19 represents a Green function of the complex propagation function xcex3 (xcfx89j). The propagation function describes how the electromagnetic field varies, both in terms of (field) strength and phase angle, as a function of the (angular) frequency xcfx89j. The mentioned constant C1 is usually set equal to 1/(16xc2x7xcfx802).
By making suitable use of a weighting factor, it is possible to correct most of the imaging aberrations. For the first time, therefore, accurate imaging becomes possible making use of a back-projection algorithm in the case of imaging on the basis of diffusion data.
The weighting factor W should be chosen such that the shape and location of transitions or objects (in fact the secondary field or radiation sources) found with its aid correspond to the actual shape and location of the transitions and the objects. A procedure for achieving this will be described later.
It is possible that a different value for the weighting factor W will have to be used for each combination of values for xsm xrn, xxe2x80x2 and xcfx89j. In other words, W is a function of these variables. The mathematical dependence of the weighting factor W as a function of xsm xrn, xxe2x80x2 and xcfx89j is constrained solely, in principle, by the purpose to be achieved. In addition, the weighting factor W can depend on the type of the measuring device used, on the base material of the surveyed site and possibly also on other conditions.
In a particular embodiment of the algorithm according to the invention, the weighting factor W, for at least some of the values of xsm, xrn, xxe2x80x2 and xcfx89j, substantially takes the form
W(xsm, xrn, xxe2x80x2, xcfx89j)=C2xc2x7(|xsmxe2x88x92xxe2x80x2|xc2x7|xxe2x80x2xe2x88x92xrn|)ndxc2x7(xcex3(xcfx89j))nxcex3, 
where
C2 is a constant,
nd is a real number between xe2x88x922 and +5, and
nxcex3 is a real number between xe2x88x924 and +4.
Experiments have shown that the mathematical form of the weighting function depends solely on the type of measuring device and not on the characteristics of the experimental data. It is therefore possible, for each type of measuring apparatus, with the aid of a few simple experiments as will be described hereinafter in detail, to determine in a suitable manner a weighting function W of the above-described form.
In a preferred embodiment, nd is between 2.5 and 3.5, and nxcex3 is between 1.5 and 2.5. More preferably, nd equals 3 and nxcex3 equals 2. This form of the weighting function is suitable, for example, for so-called NanoTEM equipment. In a similar manner, a suitable weighting factor W can be chosen for other measuring equipment.
The invention also relates to the use of an algorithm according to the second aspect of the invention in the determination of electromagnetic contrast information of a site with the aid of measurements using electromagnetic fields or radiation. To this end, the algorithm can be held in compiled or uncompiled form in the memory of an electronic arithmetic unit or be recorded on a carrier of digital data, such as a diskette, magnetic tape or hard-disk unit. It is also possible to process the data by means other than an electronic arithmetic unit, for example by hand. From an efficiency point of view, the latter is not recommended.
According to the second aspect, the invention further relates to a method of determining electromagnetic contrast information of a site according to claim 16.
Preferred embodiments of the method according to the second aspect of the invention are described in claims 17 and 18.
Additionally, the invention according to the second aspect relates to a method of determining the weighting factor W applicable to a measuring device, as described in claim 19. The term measuring device here relates to a measuring device for determining electromagnetic contrast information, for example a subsurface measuring device or a pipeline measuring device.
In practice, for example, if a subsurface measuring device is to be tuned, a known object is buried in a test field at a known position, or the electromagnetic characteristics of the soil of the test field in question are known by some other way known in the prior art. This method according to this second aspect of the invention then involves, first of all, a determination, by means of the subsurface measuring device, of the electromagnetic experimental values of the soil of the test field. Then values are chosen for the numbers nd and nxcex3 of the weighting factor W on the basis of experience or expectation. With the aid of the weighting factor W thus chosen, the experimental values are processed to yield the contrast information and thus an image of the soil including the shape and the location of the buried object.
If the observed values for the shape and the location of the buried object do not correspond to the actual situation, that is not with the desired accuracy, the weighting factor W is attuned to reality via a choice of other, more suitable values for the numbers nd and nxcex3. Then the experimental values are again processed to yield electromagnetic contrast information and therefore an image of the soil. This image is again compared with reality.
Should the observed image still not correspond, with the desired accuracy, with reality, the values of nd and nxcex3 are again adjusted, the measured values are processed etc., until the desired accuracy has been achieved. It should be noted that there are limits to the accuracy that can be achieved by this procedure. In fact, a separate weighting factor should be determined for each point to be surveyed in the site, for each source position, for each receiver position and for each frequency used, said weighting factors together forming an extremely complicated function. In practice this is not feasible. This method provided according to the second aspect of the invention provides the option of finding a good approximation to this weighting function in a simple manner.
In a comparable manner, a method of this type can be used for determining, for example, the position and thickness of any corroded patches or other discontinuities in a pipeline which is at least partly made of metal. To this end, the position and thickness of corroded patches, i.e. the electromagnetic contrast characteristics, in the pipeline are determined beforehand in some other way known in the prior art. Then the pipeline, which may or may not be buried, is surveyed by means of the measuring device, the weighting factor W being adjusted in the process until the observed contrast characteristics or corroded patches correspond to those known.
According to the second aspect, the invention further provides measuring devices for determining, with the aid of electromagnetic radiation or fields, electromagnetic contrast information of a site as described in claims 20 and 21.
In a preferred embodiment of the measuring device according to the second aspect of the invention, the weighting factor W used for that measuring device is determined making use of a method suitable therefor according to the second aspect of the invention. The weighting factor W found can be applied by using a separate electronic arithmetic unit to process the measured data determined by the measuring device. Alternatively, it is possible for the processing of the measured data with the aid of the weighting factor to be performed by an electronic arithmetic unit integrated in the measuring device. This permits an in situ judgement on whether supplementary measurements are necessary. As a result, the entire system becomes more flexible and more efficient.
The second aspect of the invention also relates to a computer program which can be loaded into a computer, such that the computer thus programmed is suitable for performing an algorithm according to the invention.
Moreover, the invention according to its second aspect relates to a computer program which can be loaded into a computer, such that the computer thus programmed is suitable for implementing a method of determining electromagnetic contrast information according to the second aspect of the invention.
In this context, it is possible for the further processing of the observed data likewise to be carried out with the aid of modules of the computer program which are suitable for this purpose. Thus, the observed data could be represented in a suitable form on a monitor or in the form of a printout.
According to its second aspect, the invention also relates to a computer program product according to claim 25 or claim 26. It involves a computer program product on which a computer program for performing an algorithm in, or a method of, determining electromagnetic contrast information according to the second aspect of the invention is recorded. This computer program product can, for example, comprise a magnetic tape, a diskette, a hard-disk unit, CD-ROM. In another embodiment, the computer program product comprises part of the read-only, non-erasable memory section of a computer or subsurface measuring device, one possible option being a so-called plug-in card.
The invention, according to the second aspect, further relates to the use of a computer program according to the second aspect of the invention. One possible option involves loading and subsequently using a computer program according to the second aspect of the invention in a computer or in a subsurface measuring device.
Another conceivable possibility would be to dispatch unprocessed measured data from a particular location to some other location where a computer program according to the second aspect of the invention has been loaded into a computer. There, the unprocessed data are processed with the aid of the computer program according to the second aspect of the invention, following which the process data can be dispatched to the location where they are required.
Transmission of the unprocessed and/or the processed data can be effected in a variety of ways. For example, the data can be recorded on an information carrier such as a magnetic tape, diskette or CD-ROM, which is then dispatched. It is also possible to dispatch the data, in digitized form or otherwise, via a network such as a telephone line or the internet.
A third aspect of the invention relates to a method of carrying out a subsurface survey, comprising the steps of generating one or more electromagnetic pulse signals, emitting the one or more pulse signals into the subsurface, receiving one or more response signals associated with each pulse signal, sampling the one or more response signals, converting each response signal sample into digital form in order to obtain a response signal word, and processing the response signal words to obtain an image of the structure of the subsurface. The invention, according to its third aspect, also relates to an apparatus for implementing this method.
A method of this type is known from the prior art, for example from International Patent Application PCT/US88/03196. Owing to limits to the speed of equipment (A/D converter) for converting an analog response signal value into a digital response signal value, and also owing to limits to the speed of equipment for processing the digital response signal words, the known method involves subsampling. Said subsampling implies that one sample is taken of each response signal of a series of successive response signals, the sampling period being somewhat larger than the response signal period. The series of response signal samples thus obtained forms a complete sample of one of the response signals, faithfully reproducing the original analog response signal. Said subsampling makes it possible to sample a high-frequency signal at a low frequency, thus enabling processing of the response signal words to be performed within the technical capabilities of the available equipment.
In principle, the electromagnetic pulse signals can be emitted into the subsurface both from the surface (above water or under water) and from a hole in the subsurface. The response signals can likewise be received both at the surface (above water or under water) and in the hole in the subsurface.
The known method has a number of drawbacks.
A first drawback is that a complete series of response signal samples can only be obtained on the basis of a large number of response signals, at a relatively high cost in terms of time and energy, since each response signal sample requires a pulse signal, resulting in a response signal, to be emitted into the subsurface.
A further drawback is that the successive response signals, of each of which one sample has to be taken each time, must be identical to a high degree, which in turn places high requirements on the uniformity of the pulse signals which give rise to the response signals, if it is assumed that the characteristics of the subsurface will not change during the time required for the series of response signal samples. At the same time, the repeat frequency of the pulse signals should be highly constant or at least be synchronized with the sampling frequency.
Another drawback is that the A/D converter of the known device performs sampling which is independent of the amplitude of the response signal, which means that for small amplitudes the amplitude accuracy obtained is relatively poor, thereby impairing the quality of the subsurface structure image ultimately aimed for.
If a higher accuracy, (e.g. 16 or 32 bits) is desired, which will generally be the case, the conversion rate of the A/D converter available for this purpose decreases, which results in the need for the abovementioned subsampling. Conversely, a relatively high conversion rate results in a relatively low accuracy (e.g. 8 bits).
It is an object of the third aspect of the invention to eliminate the abovementioned drawbacks or at least considerably reduce them. This object is achieved by a method of the above-specified type, which is characterized according to claim 27. An apparatus according to the third aspect of the invention is characterized according to claim 31.
In the method and apparatus according to the third aspect of the invention, a plurality of sampling operations are performed for each response signal, fewer response signals therefore being required to obtain a complete series of samples which adequately describe a response signal. Preferably, all those response signal samples are taken from each response signal that are necessary for a substantially complete characterization of the response signal. This permits very high savings in time and energy. Moreover, it is not necessary for the pulse signals to be uniform relative to one another or for the repeat frequency of the pulse signals to be constant or to be synchronized with the sampling frequency.
To allow the response signal words to be processed at a lower rate despite the high sampling frequency chosen, a number of successive response signal words are buffered to form a data word, the data word subsequently being processed to obtain an image of the structure of the subsurface.
In a preferred embodiment of the method and apparatus according to the third aspect of the invention, the amplitude of each response signal while being received is adjusted to the maximum amplitude applicable to the sample, which means that good amplitude accuracy is achieved even at high conversion rates and/or with A/D converters having limited accuracy for relatively small amplitudes.